Reports with Subreports problem (Access 2000)

I have a report that contains 3 sub reports. At the end of the parent report I sum a value located in each of the sub reports. If one of the sub reports contains no data, the value no longer sums. What is the correct way to handle this scenario so I will always get a sum, even if all three sub reports are null? (In that case, I would want a sum of zero)